As treatment for improving the adhesion between an organic film and a metal material like steel sheet, zinc-based plated steel sheet, zinc-alloy sheet, tin-plated steel sheet, aluminum alloy sheet, chromate treatment, phosphate treatment, silane coupling treatment, etc. have been known previously.
Among these, chromate treatment has been broadly utilized in the fields of home electrical appliances, building materials, vehicles, metal containers, etc. due to its superior corrosion resistance and adhesion, but the possibility of the toxic substance of hexavalent chrome contaminating the soil by leaching into it at the time of disposal has been pointed out, so the industry in Europe etc is already moving in the direction of eliminating chromate treatment.
In metal materials for container use, chromate treatment of a type treating tin-plated steel sheet by cathodic electrolysis in an aqueous solution of sodium bichromate or in an aqueous solution of anhydrous chromic acid not containing any fluoride so as not to leave hexavalent chrome in the film is being utilized.
However, in chromate treatment of a type where the treated layer does not include hexavalent chrome, the treatment solution itself contains hexavalent chrome, so the hexavalent chrome has to be rendered harmless for treatment of the wastewater and gas emissions. For this reason, from the viewpoint of the environment load, surface treatment where the treatment solution does not include hexavalent chrome either is desirable.
Further, fluorine, boron, nitrate nitrogen, etc. are also not preferable from the viewpoint of the load on the environment. In the future, the industry will be moving in the direction of toughening the emission standards. Therefore, the treatment solutions for metal materials for container use preferably also do not contain the above substances.
Therefore, as one measure for reducing the load on the environment, there is elimination of the use of chrome. As chrome-free surface treatment for a container material, there is an example of providing on a tin-plated surface of a tin-plated steel sheet an organic-inorganic composite coating comprising an organic compound having carbon as its main ingredient and an inorganic phosphorus compound. This is described as an example of a method of surface treatment of a tin-plated steel can superior in corrosion resistance and coating adhesion in Japanese Patent Publication (A) No. 11-264075.
As a surface treatment solution for an aluminum can or tin-plated drawn and ironed can before coating and printing, a drawn and ironed can surface treatment solution containing phosphoric acid ions and a zirconium compound and titanium compound in at least one type and containing and containing an oxidizing agent and fluoric acid and a fluoride in at least one type is disclosed in Japanese Patent Publication (A) No. 7-48677.